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Abstract. Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper ...

Climatic Change (2012) 110:709–719 DOI 10.1007/s10584-011-0121-x

Why is Indian Ocean warming consistently? Suryachandra A. Rao · Ashish R. Dhakate · Subodh K. Saha · Somnath Mahapatra · Hemantkumar S. Chaudhari · Samir Pokhrel · Sobhan K. Sahu

Received: 26 July 2010 / Accepted: 18 May 2011 / Published online: 10 June 2011 © Springer Science+Business Media B.V. 2011

Abstract Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air–sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback

S. A. Rao · A. R. Dhakate · S. K. Saha (B) · S. Mahapatra · H. S. Chaudhari · S. Pokhrel Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune, 411 008, India e-mail: [email protected] S. A. Rao e-mail: [email protected] A. R. Dhakate e-mail: [email protected] S. Mahapatra e-mail: [email protected] H. S. Chaudhari e-mail: [email protected] S. Pokhrel e-mail: [email protected] S. K. Sahu Pukyong National University, Busan, South Korea e-mail: [email protected]


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mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP.

1 Introduction Sea surface temperature (SST) of tropical oceans is one of the important parameters which determine the amount of evaporation, convection and thereby precipitation over tropical region. There are certain regions over tropical oceans, where SST values are greater than 28◦ C and are referred as “warm pools”. Many studies show that the minimum SST required for generation of active convection is 28◦ C (Gadgil et al. 1984; Graham and Barnett 1987), which is also one of the favorable conditions for the formation of tropical cyclones (Gray 1975). The most intensive air–sea interaction takes place in the Indo-Pacific warm pool (IPWP), which is also the largest body of warm water in the world and a region of active convection (e.g. Saraswat et al. 2007). A large part of the tropical Indian Ocean has SST higher than 28◦ C and is termed as the Indian Ocean warm pool (IOWP). This warm pool has a lot of significance over Indian Ocean, e.g., the Indian summer monsoon onset vortices form over its warmest waters (Joseph 1990). Most part of the IOWP is characterized by a low value of outgoing long wave radiation (OLR) (below 240 W m−2 ) indicating deep convection (Lukas and Webster 1988). Using climatology of Levitus (1982), Vinayachandran and Shetye (1991) examined the structure of the IOWP and compared it with the West Pacific Warm pool (WPWP; Cravatte et al. 2009). They found that even though the Pacific warm pool is larger and warmer, a peculiarity of the warm pool in the Indian Ocean is its seasonal variation. Their estimation showed that the surface area of the IOWP changes from 24 × 106 km2 in April to 8 × 106 km2 in September due to interaction with the southwest monsoon. Cheng et al. (2008) examined the long term trends of sealevel height in the IPWP regions using merged altimetry data and studied possible mechanisms using multiple data sets. They found that the sea level in IOWP and WPWP has distinct inter-annual variability related to ENSO event. Furthermore, Zhang et al. (2009) have found that increasing trend of easterly wind anomaly and the southerly wind anomaly over the eastern equatorial Indian Ocean causes anomalous westward and northward displacements of the eastern Indian warm pool respectively and this helps in the formation of the Indian Ocean Dipole (IOD). Ajayamohan and Rao (2008) have shown that the IOD events have increased in the last decades as a result of Indian Ocean warming. Alory et al. (2007) estimated the linear trends in oceanic temperature in Indian ocean, using Indian Ocean Thermal Archive (IOTA), a compilation of historical temperature data above 1,000 m in the Indian Ocean. They found a widespread surface warming over Indian Ocean. IOTA shows a subsurface cooling in the tropical Indian Ocean, which corresponds to a

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shoaling of the thermocline. Modeling studies show that this trend in the thermocline over tropical Indian Ocean could be associated with the observed weakening of the Pacific trade winds, which might have been transmitted to the Indian ocean by the Indonesian through-flow. It has also been found that the warm pools of the tropics have been expanding in area during the last 150 years and the rate of expansion is increasing in the last few decades (e.g. Webster et al. 2006). So, temperature, size and positioning of the IPWP are gradually changing, which can have a profound effect on global climate (Cane and Clement 1999). Many previous studies have highlighted the Indian Ocean warming (e.g. Levitus et al. 2000, 2005; Willis et al. 2004; Alory et al. 2007) and it is believed to be due to changes in net air–sea heat flux induced by global greenhouse gas concentration. However, there is no quantitative measure of greenhouse induced warming and it is also not clear whether the ocean dynamics are involved in the warming and expansion of IOWP. Is the increased frequency of IOD linked with the above mentioned changes in IOWP? Here we propose a coupled positive feedback mechanism for the observed expansion and warming trend of IOWP. We have hypothesized that the occurrence of IOD events is dynamically linked with the changes in IOWP. This paper has been organized in the following order: Section 2 describes the data used in this study; results and discussion are given in Section 3 and the results of this study have been summarized in Section 4.

2 Data used In this study, we have used NOAA NCDC (National Oceanic and Atmospheric Administration National Climate Data Center) ERSST (extended reconstructed SST) version 3b data for the period 1950–2009 (Smith et al. 2008; Reynolds et al. 2002). SST anomaly and trend are calculated for the period 1950–2009. In order to identify a time series representing the IOWP warming and expansion, we followed several methods. EOF analysis of SST anomaly (1950–2009) for tropical Indian Ocean reveals that the first EOF mode (45.81%) explains the strong warming trend in the Indian Ocean. Since EOF analysis reveals only standing modes, the warm pool expansion cannot be explained by the principal components (PCs) of these EOFs. Alternatively we have defined an index representing IOWP area (>28◦ C) in the domain 30◦ E–120◦ E and 30◦ S–30◦ N, excluding south China sea region. This index is highly correlated with the first PC (≈0.8) and therefore can explain both the warming and expansion of IOWP. The net surface heat flux data are taken from objectively analyzed (OA) air–sea flux data (Yu et al. 2008; Yu and Weller 2007) for the period 1984–2007. Wind data used are from NCEP reanalysis (Kalnay et al. 1996) for the period 1984–2007 at 1,000 hPa. High resolution AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data) merged sea surface height (SSH) data for the period 1992–2007 are used (http://www.aviso.oceanobs.com/). NOAA interpolated Outgoing Longwave Radiation (OLR) dataset (Liebmann and Smith 1996) has been used for identifying high convective regions. For the estimation of warming trend due to advection, the sub-surface currents and temperature of top 30m layer from various data assimilation products are used: (i) NCEP Global Ocean Data Assimilation System (GODAS), (ii) Simple Ocean Data Assimilation (SODA) (Carton and Giese 2007) and (iii) Estimating the Circulation and Climate of the


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Ocean (ECCO) (http://www.ecco-group.org/las/v6/dataset/dataset?catitem=546) are used. Since the reliable air–sea flux data are available from 1984, we have restricted our analysis for this period and the experimental results obtained in this study reflects the trends in last two decades.

3 Results and discussions The mean SST for the period 1950–2009 is shown in Fig. 1a. Water warmer than 28◦ C (representing warm pool) covered most parts of the Bay of Bengal, east and south east Arabian sea and a large part of the equatorial Indian Ocean. The annual cycle of the IOWP area (monthly climatology mean for the period 1950–2009; Fig. 1b) shows a strong seasonal cycle with higher values in the pre-monsoon summer months and lower values during the monsoon season. The IOWP area becomes maximum (about

Fig. 1 a Climatological mean SST (1950–2009) from extended Reynold’s SST data. b The monthly mean IOWP area climatology (1950–2009), showing the annual cycle. Warm pool is defined as oceanic region with SST >28.0◦ C. The vertical axis represents area of the warm pool in 106 km2 . c SST anomaly trend per decade for the data period 1950–2009. d Time evolution of IOWP area anomaly for the data period 1950–2009, showing the inter-annual variability and increasing trend

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Fig. 2 Warm pool area during 1950–1960, 1975–1985, 1998–2008 with SST a above 28.0◦ C, b above 28.5◦ C and c above 29.0◦ C

25 × 106 km2 ) in the month of April and minimum (about 10 × 106 km2 ) during the months of August and September. These values are higher as compared to those reported by Vinayachandran and Shetye (1991). This indicates that IOWP area has expanded in recent decades. It may be noted that the warming trend is not uniform throughout the domain but the maxima lies over south of the equator (Fig. 1c). The warming trend in the tropical Indian Ocean in recent decades is the strongest compared to any other tropical basin (Du and Xie 2008). An increasing trend of IOWP area anomaly along with strong inter-annual variations are evident in the last 60 years (1950–2009, Fig. 1d). Increasing trend of area anomaly indicates the continuous expansion of IOWP. A careful observation further shows that, the timing of strong inter-annual variations are happen to be ENSO year. Since 1980, IOWP area has expanded significantly (Fig. 1d). Furthermore, the relative expansion of 29◦ C water is much more than the 28◦ C and 28.5◦ C water (Fig. 2). A strong positive correlation is found between the IOWP area anomaly and the SST anomaly over IOWP in recent decades (1980–2009), as shown in Fig. 3a. The correlation is higher over the western Indian

Fig. 3 a Correlation between IOWP area anomaly and SST anomaly (1980–2009). b Estimated SST anomaly trend per decade using objectively analyzed net heat flux data for the period 1984–2007


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Ocean, particularly south of the equator and regions just south of Indian landmass. This illustrates westward expansion of IOWP in recent decades as compared to the mean warm pool area. As the warm pool area is restricted between 10◦ S to 10◦ N (Fig. 1a) and expanded in recent decades (Fig. 2), the association between warm pool area anomaly and SST is confined mainly to these region. The warming of IOWP might have been caused by changes in net surface heat flux or/and due to ocean advection. In order to understand the possible reason for consistent warming and expansion of Indian Ocean, the relative role of air–sea fluxes and oceanic advection are investigated. The SST tendency equation is given by   ∂T ∂T ∂T ∂T Qs =− u +v +w + +D (1) ∂t ∂x ∂y ∂z ρC p h The first and second term on the right hand side represent the temperature tendency due to advection and net surface heat flux. The third term (D) represents the tendency due to diffusion processes. Here, u, v, w are the speed of zonal, meridional and vertical component of current, Qs is the surface (air–sea) net heat flux, h is the mixed layer depth (here fixed value of 30 m is used), ρ is the density of sea water and C p is the specific heat capacity of sea water. In order to understand the possible contribution from changes in net surface heat flux towards warming of IOWP, we have estimated SST anomaly tendency using the OA air–sea fluxes alone. The estimated trend of SST anomalies using OA net heat flux is shown in Fig. 3b. The change in surface fluxes causes cooling near the equatorial regions, particularly south of the equator, eastern Arabian Sea and northern Bay of Bengal. Further analysis using individual components of surface net heat flux reveals that the radiative (long wave and short wave) fluxes tend to increase the SST whereas the sensible and latent heat fluxes tend to cool the SST, which are in agreement with Alory and Meyers (2009). The latent heat flux played a major role in cooling the Indian Ocean, especially south of the equator. Therefore, the cooling due to net heat flux is mainly contributed by latent heat flux, which shows maximum increasing trend over east equatorial Indian Ocean (Yu and Weller 2007). The OA surface fluxes not only fails to explain the observed warming but it acts in the opposite direction. The correlation between IOWP area anomaly and net heat flux anomaly is weak and negative (Fig. 4a), which again suggests the importance of ocean dynamics in warming and expansion of IOWP. For the estimation of temperature tendency due to advection processes, assimilated ocean sub-surface data (GODAS, ECCO, SODA) are used. Maximum warming trend are found at south of the equator in all of these data sets (e.g., Fig. 4b using SODA data for the period 1984–2007). Each of these data sets suffers from certain biases due to limitations of their own assimilation scheme and lack of observational data feed into the assimilation system. Therefore, quantification of warming could not be made due to lack of reliability in these data sets. Above analysis suggest that ocean dynamics have contributed towards the warming in the tropical Indian Ocean region. The SSH anomalies are strongly correlated (>0.5) with the warm pool area anomaly (Fig. 4c) and the largest positive correlation is seen over western part of Indian Ocean. It may be noted that the slope of positive maxima correlation is from north-west to south-east (south-west to north-east) in the south (north) of the equator, which suggests Rossby wave pattern. The lead/lag correlation, averaged over 8–10◦ S as a function of longitude (Fig. 4d) suggests that an

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Fig. 4 a Correlation between objectively analyzed net heat flux and warm pool area anomaly for the period 1984–2007. b Temperature trend (in ◦ C decade−1 ) due to advection using top 30 m SODA data (1984–2007). c Correlation between sea surface height anomaly and warm pool area anomaly for the period 1992–2007. d Lead/lag correlation between warm pool area anomaly and merged sea surface height anomaly (1992–2007). e Regressed wind anomaly with the warm pool area anomaly (vector plot), regressed wind speed anomaly with warm pool area anomaly (color shading) and wind curl (green contour) for the period 1984–2007. f OLR anomaly trend per decade for the period 1974–2009

increase in IOWP area is related to deepening of thermocline and advection of warm water to the west. The correlation maxima move from 95◦ E to 45◦ E with a speed of about 12 cm s−1 (total time span of about 17 months), which is consistent with the observed speed of westward propagating Rossby wave (10 cm s−1 ) at 12◦ S (Rao et al. 2002). Therefore, the structure and speed of westward propagating positive correlation maxima suggest that oceanic Rossby waves are involved in causing the Indian Ocean Warming. Similar correlation patterns are also observed using high


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pass filtered warm pool area anomaly and SSH anomaly with cut of frequency of 30 months. We have also used longer period of SODA SSH data (Carton and Giese 2007) and the results are found to be in good agreement with that from AVISO. In addition to that, all the three reanalysis data sets (GODAS, ECCO, SODA) show similar correlation patters as shown in Fig. 4c and d (figures are not shown). Region of significant positive correlations in Fig. 4c coincides with the southern tropical Indian Ocean thermocline ridge (Xie et al. 2002). Also, this is the region where strong air–sea coupling takes place. In order to investigate the possible role of ocean dynamics linked with the surface winds, we have regressed Indian Ocean warm pool area anomalies with zonal and meridional component of wind and wind-speeds. It is noticed that for the major portion of the Indian Ocean, winds are weakening. Strong easterly wind anomalies are observed along the equator in association with increasing warm pool area (Fig. 4e). Similarly, the decreasing trends of OLR anomalies (Fig. 4f) over west central equatorial Indian Ocean implies increasing trend of cloudiness, i.e. the increasing trend of convective activities. It is further found that, use of warm pool area index based on warm pool area of the whole domain (i.e. 30◦ E–120◦ E and

Hypothesis Indian Ocean Warming /

Deepening of thermocline ridge in the western equatorial Indian Ocean by downwelling Rossby waves (Fig. 3c, 3d)

increase of warm pool area (Fig. 1c, 1d)

Enhancement of convection over west central equatorial Indian Ocean (Fig. 3f)

Coupled Positive Feedback Strengthening of anti-cyclonic wind stress curl on either side of the equator (Fig. 3e)

Strengthening of anomalous equatorial easterlies Frequent IOD events

(Fig. 3e)

(Ajayamohan and Rao, 2008)

Fig. 5 A schematic diagram of coupled ocean–atmosphere feedback mechanism (positive feedback) for the warming and expansion of IOWP

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30◦ S–30◦ N) including south China sea does not change the result. From the above analysis, the following interpretation can be made: 1. Due to warming of IOWP (over central equatorial Indian Ocean), the anomalous convection is shifted towards west central equatorial Indian Ocean. 2. As a result of enhanced anomalous convection over west central equatorial Indian Ocean, strong anomalous easterlies are observed over the equator and twin cyclones startling the equator are noticed in the western Indian Ocean, as a consequence to atmospheric Kelvin and Rossby wave response to strong convection at the equator. This pattern depicts a typical Matsuno–Gill pattern (Matsuno 1966; Gill 1980). Based on the above observations, we put forward the following hypothesis to explain the possible reasons for warming and expansion of IOWP. Under Indian Ocean warming conditions, the enhancement of convection takes place in west central Indian Ocean. As a response to this enhanced convection, anomalous easterlies are observed along the equator (Fig. 4e). Anomalous easterlies in summer and fall season can excite Indian Ocean dipole (IOD) event (Murtugudde et al. 2000; Rao et al. 2002). Under these circumstances, the frequency of IOD events increases (Ajayamohan and Rao 2008). In the last two decades seven positive IOD events occurred in the Indian Ocean region (Rao et al. 2010). At the same time, the wind stress curl in south-east and north-east equatorial Indian Ocean becomes anticyclonic (Fig. 4e) and that anti-cyclonic wind stress curl can trigger down-welling oceanic Rossby waves. This anomalous down-welling oceanic Rossby waves deepen the thermocline and result in advection of warm waters towards western Indian Ocean, resulting in more warming and expansion of IOWP. This coupled feedback mechanism is shown in Fig. 5. It may be noted that the anti-cyclonic wind stress curl in north-east equatorial Indian Ocean is weaker than that of south-east equatorial Indian Ocean and the Rossby wave propagation in north of the equator is hindered due to Indian land-mass. On the other hand, the Rossby wave can propagate freely in the south of the equator. Therefore, the structure of Rossby wave is more clear in the south of the equator than that in the north (Fig. 4c).

4 Summary and conclusions It is observed that IOWP is consistently warming and expanding. It was found that the changes in net surface heat flux alone can not explain the observed warming trend of IOWP and the changes in heat fluxes are such that, it tends to cool the ocean. The correlation between sea surface height anomaly and warm pool area anomaly reveals an oceanic Rossby wave signal and hence ocean dynamics played important role in the warming and expansion of IOWP. We have also used several ocean data assimilation products to estimate temperature trend due to advection. All of these data sets confirms the importance of oceanic advection processes in Indian Ocean warming. We proposed a coupled ocean-atmosphere feedback mechanism through which the observed warming and expansion of IOWP can be explained. This mechanism suggests an enhancement of convection in central Indian Ocean in an warming environment and as a result, anomalous easterlies are strengthened, which can excite more number of IOD events. Also, the wind stress curl in south-east and


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north-east equatorial Indian Ocean becomes anti-cyclonic, triggering downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian Ocean. This results in more warming of Indian Ocean (particularly south of the equator) and westward expansion of IOWP. Thus, the coupled ocean-atmosphere system works as a positive feedback for consistent warming and expansion of IOWP. This coupled feedback mechanism is illustrated schematically in Fig. 5. This study is constrained by the lack of oceanic observations and limitations in the data assimilation schemes and hence could not resolve the quantitative role of oceanic advection for warming of IOWP. Therefore, it is suggested that a good oceanic observation network is very much required for resolving the details. Furthermore, a carefully designed coupled model experiments may provide more details for the positive feedback as suggested in the study. This study explains the consistent warming in last two decades. It will be interesting to find out the reasons for warming of the Indian Ocean in earlier decades as well. It is also interesting to find out how the consistent warming of the Indian Ocean can influence the largescale atmospheric circulation, e.g. Indian summer monsoon. Acknowledgements This work is supported by Ministry of Earth Sciences, Government of India through INCOIS (Indian National Centre for Ocean Information Services) by sponsoring the project entitled Variations of Indian Ocean warm pool and its association with Indian monsoon. All data sources are duly acknowledged.

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